High-voltage metal-oxide-semiconductor (HVMOS) devices are widely used for many types of high-voltage circuits such as input/output circuits, CPU power supply circuits, power management systems, AC/DC converters, etc. The commonly seen HVMOS devices include lateral-diffused metal-oxide-semiconductor (LDMOS) devices and double-diffused drain MOS (DDDMOS) devices. HVMOS devices may include lightly doped well regions in order to enhance the breakdown voltages.
HVMOS devices need to sustain high voltages. Accordingly, the gate dielectrics of the HVMOS devices also need to endure high gate-to-drain voltages. Conventionally, the gate dielectrics of HVMOS devices are thick oxides that have greater thicknesses than the gate oxides of low-voltage MOS (LVMOS) devices such as core devices, which LVMOS devices may also be formed on a same chip as the HVMOS devices. The conventional formation processes of a HVMOS device and an LVMOS device may include forming a sacrificial oxide layer and a sacrificial nitride layer in both a HVMOS region and a LVMOS region, removing the sacrificial oxide layer and the sacrificial nitride layer from the HVMOS region and re-growing a thick oxide layer in the HVMOS region, removing the sacrificial oxide layer and the sacrificial nitride layer from the LVMOS region and re-growing a thin oxide layer in the LVMOS region (and possibly over the thick oxide layer also), and then forming a polysilicon layer over the thick oxide layer and the thin oxide layer. The above-mentioned layers are then patterned to form the gate stacks of the HVMOS device and the LVMOS device.